Pregnant women who use mobile phones are more likely to have children with behavioural problems, a shocking study has found.

Using handsets just two or three times a day is enough to raise the risk of hyperactivity and emotional problems.

Letting children use mobiles before the age of seven also puts them at risk, scientists warn.

The research is the latest in a series of health fears linked to mobile phones and yet another worry for expectant mothers, who have already been warned about drinking, smoking, pesticides, food allergies and stress.

The study follows a finding by the official Russian radiation watchdog that the danger posed by mobile phones is "not much lower than the risk to children's health from tobacco or alcohol".

The UK's Health Protection Agency said the study's findings were unexpected and highlighted the need for caution over mobiles. But it stopped short of telling pregnant women not to use them.

A spokesman said: "Its findings need to be investigated thoroughly. There may be another cause for the effect observed."

The agency has already warned against "excessive" use of mobile phones by children.

The study, the first of its kind in the world, covered more than 13,000 women.

It found that those who used phones while pregnant were 54 per cent more likely to report behavioural problems in their children, including hyperactivity and emotional and relationship difficulties.

Problems were even more likely among children whose mothers had used phones while pregnant and who had used mobiles themselves before the age of seven.

They were 80 per cent more likely to suffer from behavioural difficulties compared to youngsters who had not been exposed to mobile phone use at all.

They were 25 per cent more at risk of emotional problems, 24 per cent more likely to have difficulties relating to other children, 35 per cent more likely to be hyperactive and 49 per cent more likely to have problems with behaviour.

The risks increased with the amount of phone use and potential radiation.

Experts said the results were particularly significant as one of the study's authors has been a leading sceptic over claims that mobile phones were a health risk.

Professor Leeka Kheifets of the University of California, Los Angeles, had previously written that there was no proof of "any adverse health effect".

But she and her three co-authors concluded that there did appear to be an association between mobile phone exposure and behavioural problems.

They warned, however, that there were other possible explanations, including that mothers who were frequently on the phone through pregnancy might continue the pattern after birth, spending less time with their babies.

Previous studies have suggested a link between children being left unattended for long periods and later behavioural problems.

The research, conducted jointly with academics in Aarhus, Denmark, is due to be published in the respected medical journal Epidemiology in July.

Radiation from mobile phones is thought to penetrate only one or two centimetres into the skin and not to reach the unborn child.

But previous studies have shown it can affect levels of melatonin, a sleep- controlling hormone that mothers pass to unborn babies through the placenta.